flooding in the USA

The complete guide to preventing flood damage

This article explains what to do and what steps to take if your house is flooded or at risk from flooding.

Floods seem to be occurring in Ireland with greater frequency than ever before, affecting just about every part of the country.

If you live in a flood-prone area you should make the necessary preparations without delay.

This section of our WeatherSafe site provides information on the wide range of permanent measures that can be undertaken to improve the flood resistance of new and existing buildings. These measures can help to reduce the extent of flood damage, thereby reducing repair costs and speeding up building restoration times.

Professional advice should always be obtained before any improvement measures contained within this guide are undertaken. When handling building materials care should also be taken to follow manufacturers health and safety instructions. Some flood resistance measures discussed here may require building control approval, depending on the type of property concerned.

flood damage


A number of different external wall types are found within domestic and small business properties across Ireland including solid masonry walls, cavity walls, timber-framed walls, and half-timbered walls. Before considering flood improvement measures it is necessary to establish the type of wall, if necessary by seeking professional advice.

The outside face of external walls

The outside face of most external walls is normally constructed of brickwork, which is either left exposed (fair faced), rendered and or/painted. For existing properties, cracks within the brickwork joints or render below the expected maximum flood level should be repaired to reduce the amount of seepage through the wall. A variety of water-resistant paints and coatings (or tanking) are available that can help prevent floodwater soaking into the external face of the wall, thus allowing the wall to dry out more quickly.

Specialist advice must always be obtained to ensure that the most appropriate system is chosen for the property. Coatings should be applied to 500mm above the maximum expected level of flooding. Any measures to improve water resistance must be compatible with the existing wall materials and must allow adequate water vapour transmission to avoid trapping moisture within the wall.

For new properties the choice of materials including bricks, mortars and renders should be carefully considered to limit water penetration during flooding. Generally denser materials will provide greater flood resistance.

The outer leaf of a timber-framed wall is normally formed from masonry with or without a sand/cement render finish. Other cladding types, such as tiles, timber, plastic and metal sheeting, can also be found but these are often above the first floor level.

Flooding is unlikely to damage these types of cladding, although some timber cladding boards may need to be replaced. All fixings should be corrosion resistant especially where there is a risk of seawater flooding.

The inside face of external walls

To improve the flood resistance of internal wall surfaces (assuming that floodwater will enter the building), there are a number of measures that can be taken depending on the type of wall and surface finish as follows.

Gypsum plaster

Solid and cavity masonry walls normally have an internal finish of plaster applied directly to the brick or blockwork. Gypsum based plaster on masonry walls is likely to deteriorate following repeated exposure to floodwater.

It is normal practice for gypsum plaster to be removed from the wall face after floodwaters have subsided to a level just above the flood tidemark

. This is normally carried out regardless of the plasters condition to allow the underlying masonry to dry out. Consideration may be given to replacing gypsum plasters with more flood resistant materials such as:

  • An internal water-resistant render and lime based plaster finish.
  • Ceramic tiles: particularly in kitchen areas.
  •  Hydraulic lime coatings.

Any new water resistant lining should be installed to a height of 500mm above the maximum expected flood level to prevent water being absorbed into the old lining above. For refurbishment works and new buildings it may be more cost effective to install the lining to ceiling level.

Lime based plaster

Lime based plaster products have good water resistance properties and once installed over a water resistant render, such as sand/cement or proprietary sealant, should provide effective resistance to flooding provided such finishes are applied in full accordance with the manufacturers instructions.

Following immersion in floodwater lime based plasters over a water resistant render should dry out quickly and should not deteriorate even if subjected to repeated exposure to floodwater. The water-resistant render beneath the plaster reduces water penetration into the underlying masonry, allowing the wall to dry out more quickly without the need to remove the plaster finish. Hence repair costs can be reduced.

Hydraulic lime

Hydraulic lime finishes with water resisting additives can also provide effective flood resistance for masonry walls, both internally and externally. As with lime based plaster, hydraulic lime coatings have good water resistance properties and can allow walls to dry out without the need for replacement following immersion in floodwater.

Ceramic tiles

Ceramic tiles can also provide a water-resistant surface and can reduce the volume of floodwater penetrating through to the masonry from the inside face of the wall. Tiled finishes should not need to be replaced following exposure to flooding provided a water resistant grout, suitable for the wall surface, is used in accordance with the manufacturers instructions. Tiles should be laid over a sand/cement render to provide a flat wall surface. For flood protection purposes, tiles should not be placed over gypsum based plaster.

Plasterboard (solid walls and cavity walls)

Solid masonry walls and cavity walls may have plasterboard finishes, often known as drylining. Plasterboard has low flood resistance and will almost certainly need to be replaced following exposure to floodwater. Existing plasterboard on solid masonry walls can be replaced with a water resisting liner such as a lime based plaster, ceramic tiles or hydraulic lime coating as discussed above. Where this is not practicable plasterboard sheets can be placed horizontally rather than vertically.

In the event of future flooding this can reduce the amount of plasterboard exposed to water damage, reducing the cost and time of repairs. Fixings should be of galvanised/stainless steel or copper rather than mild steel materials which may rust and cause staining of wall surfaces.

Internal finishes of timber-framed walls

There are no practical options for improving the flood resistance of the internal face of a timber-framed wall. In most cases the plasterboard will need to be removed to allow the timber frame to dry out and the plasterboard sheets themselves will also normally suffer damage if exposed to floodwater. Replacing plasterboard sheets horizontally on a timber framed wall is unlikely to be practicable due to the presence of the vapour control layer between the timber frame and the plasterboard lining.

The vapour control layer prevents condensation from forming within the centre of the wall, avoiding possible decay of the timber frame. It is therefore essential that the vapour control layer is repaired during flood damage restoration works. Replacing plasterboard sheets horizontally would create problems in providing an effective joint in the vapour control layer.

There is unlikely to be any benefit in replacing plasterboard with water resistant timber sheeting as this would still need to be removed to allow the timber frame to dry out, thus negating any potential cost savings.


Solid masonry walls

Solid masonry walls in modern properties may have an external or internal layer of insulation. Solid walls with plasterboard finishes may incorporate an internal layer of insulation between the masonry and plasterboard. Low absorption boards or semirigid self draining mineral wool batts to 500mm above the expected flood level should be considered rather than quilted insulation materials.

Insulation bonded to plasterboard will usually need replacement if exposed to floodwater. External insulation incorporating self-draining mineral fibre batts or boards or rigid plastic insulation is likely to be only temporarily affected by flooding and will ultimately dry out.

Cavity walls

Even if measures are taken to improve the water resistance of the outside face of the wall it is likely that some floodwater will seep through to the cavity, particularly in the case of long duration floods.

After repeated exposure to floodwater the steel wall ties between the internal and outer leaf of the wall may begin to corrode, particularly in cases of saltwater flooding. Wall tie corrosion can often be detected from signs of cracking within the mortar joints. Should corrosion be identified the ties should be replaced with stainless steel ties. Specialist advice from a qualified building surveyor will be required for the inspection and replacement of wall ties.

Stainless steel ties should be used in all new build properties at risk of flooding. The thermal performance of some insulation materials may be reduced following saturation from floodwater, particularly lightweight blown-in materials.

For existing properties such materials can be replaced with closed cell insulation foam, which is not water absorbent, but this can be a difficult and expensive task. Specialist advice from cavity-fill installers should be obtained to establish whether replacement is necessary and to identify the options available.

Timber-framed walls

Existing insulation within a timber-framed wall will typically consist of highly absorbent mineral wool that is susceptible to flood damage. Such materials will need to be replaced after exposure to floodwater.

Other more flood resistant insulation materials are available but it is unlikely that using such materials would provide any cost benefit as they would normally still need to be removed in order to allow the timber frame to dry out.

Replacing damaged insulation with new mineral wool is likely to be the most cost effective option after the timber frame has been allowed to dry out accepting that the material will need to be replaced should flooding re-occur. Sheathing fitted on the outside of the timber frame (normally plywood or oriented strand board) provides additional stability to the timber frame called racking resistance.

Fibreboard sheathing may become weakened as a result of flooding. If a structural assessment by a qualified engineer shows the racking resistance to be inadequate new WBP bonded plywood sheathing should be added to the inside of the frame.

The timber frame within the wall is unlikely to be adversely affected by flooding provided it dries out within a few weeks. The longer the timber remains wet the greater the risk of decay.

Steel Framed Walls

There are now a number of specialist steel framed wall systems available. Advice on what measures can be taken to improve the flood resistance of these systems should be sought from the product supplier.


Masonry walls

The flood resistance of internal masonry walls can be improved by following the principles set out relating to external masonry walls (inside face). Some partition walls in older properties may have lath and plaster linings. After exposure to floodwater linings of lath and plaster, in good condition, can dry out within a few weeks in a heated building.
However, repeated flooding will almost certainly result in decay of the lining and the need for replacement.

Stud walls

Some stud partition walls may be load bearing (supporting walls and floors above) so it is important to seek professional advice before carrying out remedial works. Stud walls, comprising of timber framing and plasterboard are vulnerable to damage from flooding.

Consideration should be given to replacing untreated softwood timber with water-resistant timber sections. Plasterboard sheets can be replaced with timber panels, using WBP bonded plywood, but this is unlikely to provide a good finish for decorations. Alternatively, plasterboard can be replaced horizontally thus reducing the amount of plasterboard replacement should flooding re-occur.

Partitions in timber-framed buildings

In timber framed houses some internal walls may be load bearing. No vapour control layer will be present and hence plasterboard sheets could be replaced horizontally rather than vertically.

Separating walls

Walls used to separate semidetached or terraced homes are designed to provide an effective sound and fire barrier. Masonry walls of brick or blockwork are likely to be of solid or cavity construction finished with plaster or plasterboard.

In timber framed houses the separating wall is also likely to be timber-framed.

These walls are finished with extra layers of plasterboard and incorporate sound absorbent material (interference with the sound absorbent material may permanently impair sound insulation properties). There are no practical measures that can be taken to improve the flood resistance of these walls.


There are three main floor types used within residential buildings and small business properties

  •  suspended timber floors,
  • solid concrete floors and
  •   suspended concrete floors.
  • Suspended timber floors

Timber joists and floorboards will normally dry out after being immersed in floodwater without any long term effects provided they have been treated with an appropriate preservative. Should joists need to be replaced they can, subject to professional advice, be supported on hangers rather than being built into walls.

This can help to minimise the risk of future distortion following wetting and drying. Additional struts or battens can also be installed between joists to reduce the risk of future distortion.

Flooring materials

Chipboard flooring usually needs to be replaced following exposure to floodwater and should be replaced with appropriately treated floor boards or WBP plywood. Removable hatches should be installed to allow access to the sub floor void and to allow the void to dry out following flooding.


Many suspended timber floors in modern buildings have mineral wool insulation between the timber joists. This material has poor flood resistance and will need to be replaced following exposure to floodwater. Other more water resistant insulation materials are available, such as low absorption boards.

Although it may be possible to re-use this type of insulation after exposure to floodwater it would be more expensive to install than mineral wool, and would need to be removed temporarily to allow the timber joists to dry out. The additional material and labour costs involved are likely to outweigh any replacement cost savings. Hence replacing damaged floor insulation with traditional mineral wool is likely to be the least cost option, accepting that this material will need replacement should flooding return.

Sub-floor void

After floodwaters have subsided water is likely to be retained below the suspended floor within the subfloor void. The sub-floor void should be cleaned of debris to allow future floodwater to be quickly pumped away and to avoid moisture retention.

In the case of refurbishment works or new properties consideration should be given to providing a fall to the sub-floor surface to an identified drainage point. For modern buildings the sub floor area is likely to be covered with 100mm of concrete or heavy-duty polyethylene sheeting with concrete or sand blinding. Any damage to these linings as a result of flooding should be repaired during the drying out period.

Solid concrete floors

Modern solid concrete floors with damp proof membranes are generally regarded as the most flood resistant floor type as they can reduce the rate of seepage into the building and are generally easier to clean and restore when compared to suspended floors. In particular solid floors do not have sub-floor voids which often require cleaning out following flooding.

However it should be noted that very few solid floors in existing properties have effective connections between the damp proof membrane (dpm) and the damp proof course (dpc) in the walls, so floodwater ingress is likely to occur at the floor/wall joint.

For new properties, or replacement floors, in areas at risk of flooding effective connections between the dpm and dpc should be provided. Older properties with solid concrete floors, especially those built before the 1950s, have no dpm beneath the concrete slab and will be less effective in reducing floodwater ingress, particularly in areas of permeable ground conditions and where the floor slabs are cracked.

Although concrete floor slabs are unlikely to be significantly damaged by flooding they can take a long time to dry out depending on the location of the (dpm). Floors with the dpm between the surface screed and the concrete slab will dry out more quickly than floors with the dpm below the concrete slab.

 Floor materials

In more modern buildings, solid floors also include a layer of insulation which can be located either above or below the concrete slab. Insulation for solid concrete floors in areas of flood risk should take the form of rigid boards with low water absorption. Some concrete floors have chipboard finishes above the concrete slab, sometimes supported on timber battens.

If exposed to floodwater it is likely that such finishes will require replacement. Consideration should be given to replacing chipboard with screed (to a minimum thickness of 65mm), tiles or WBP bonded plywood.

Underfloor heating systems can be found in some solid concrete floors, particularly in those constructed during the 1970s. Such systems may be damaged by flooding and should be checked by a qualified electrical engineer before re-use.

Suspended concrete floors

Suspended concrete floors are typically formed by in situ or precast concrete beams with infill units of lightweight concrete or clay blocks. A damp proof membrane of polyethylene directly above the floor units is normally laid to prevent moisture from the sub-floor void reaching the floor surface. Insulation of lightweight blocks or moisture resistant material is often laid between the separating layer and surface screed.

As with suspended timber floors floodwater is likely to collect in the void beneath the floor. In the case of refurbishment works or new properties consideration should be given to providing a fall to the sub-floor surface to an identified drainage point.

Solid concrete floor

Chipboard boarding is sometimes found with suspended concrete floors rather than a finishing layer of screed. As discussed above under solid concrete floors such finishes will nearly always need to be replaced following exposure to floodwater. A concrete screed of minimum thickness 65mm will provide a more flood resistant surface and if properly installed should not require replacement following exposure to floodwater, provided access panels are provided to gain access to the sub-floor void.

Concrete beams within suspended concrete floors contain reinforcement that can corrode if there is a high chloride content within the floodwater. During restoration of a property after flooding the condition of the concrete beams should be inspected for signs of corrosion.

Replacement floors

In cases of repeated flooding consideration may be given to replacing existing suspended floors with a new solid concrete floor. Solid concrete floors, provided they are properly designed to withstand the pressure of floodwater, can provide an effective seal to prevent water seeping up from the ground.

Effective connections are however required between the damp proof membrane beneath the concrete slab and the damp proof course in the walls. Solid concrete floors generally suffer less damage than suspended timber floors and are normally less expensive and faster to restore following exposure to floodwater.

Radon and landfill gas barriers

In many parts of the country ground floors need to be sealed to prevent naturally occurring radon gas, or methane or carbon dioxide from landfill sites, from seeping into the building though the ground. For solid concrete floors the radon barrier also serves as the damp proof membrane beneath the concrete slab. For suspended timber floors polythene membranes are installed below ground level.

Suspended concrete floor

In areas of high radon levels ventilation sumps are sometimes installed. Such sumps are likely to require maintenance following flooding. When undertaking any measures to improve the flood resistance of ground floors it is important not to damage such gas barriers or sumps. If in doubt seek professional advice.


For existing properties with basements specialist advice must always be obtained before undertaking any measures to prevent water ingress. Restricting ingress through internal or external tanking for example may lead to an increase in hydrostatic pressure acting on the basement walls leading to possible structural damage.

Basement construction should usually be avoided for new properties within flood risk areas.


Kitchen Cupboards

The majority of kitchen storage units are formed from chipboard and MDF with plastic coatings. Such units will almost always need to be replaced when exposed to floodwater. It is preferable to raise such units 100mm to 150mm above floor level by means of plastic legs to prevent damage during shallow flooding events.

Cupboard doors and worktops are also often made from chipboard or MDF and these will also usually require replacement if exposed to floodwater. Solid hardwood doors may dry out but warping or discolouration of the surface finish may still result in the doors needing to be replaced. If sufficient warning is provided cupboard doors may be removed in advance of floodwaters entering the property.

Water-resistant PVCu kitchen units that can be cleaned and re-used following exposure to floodwater are now available. Such systems should be considered when undertaking refurbishment works or building new properties within flood risk areas. It should be noted however that such water-resistant units would often still need to be removed temporarily following flooding to allow the floors and walls to be cleaned and dried.

Kitchen Appliances

Separate hobs and built-in ovens are preferable to combined free-standing cookers as these are mounted above floor level and may not be affected by shallow flooding. Remember that all gas and electrical appliances that have come into contact with floodwater must be inspected by a qualified engineer before they are re-used.

Washing machines and fridge freezers usually need to stand on the floor due to their weight and should not be permanently raised to provide protection against floodwater, unless specialist design measures are taken. It may be possible to empty and temporarily raise freezers on low-level blocks provided there is sufficient flood warning time.

For properties at risk of flooding separate fridge and freezer units should be considered rather than combined fridge/freezers as they would be easier to lift in the event of flooding.


Toilets and hand basins are not normally affected by flooding. However, many plastic baths have integral chipboard bases to provide rigidity. These bases can often become contaminated on exposure to floodwater resulting in the need to replace the bath unit. Higher quality bath fittings should be considered during refurbishment works or for new build properties at risk of flooding.


There are a number of external and internal door types including timber, PVCu and aluminium. Generally solid doors and frames are less susceptible to flood damage than hollow types which can fill up with contaminated water during flooding which can be difficult to drain.

Timber doors

Hollow timber doors, although less expensive than solid timber, will often de-laminate on exposure to floodwater and will usually need to be replaced. Solid timber doors are likely to be only slightly affected by flooding but some distortion often occurs and refitting is usually necessary. Such distortion can be minimised by ensuring all faces of the timber, including the bottom face, are effectively sealed using either an oil-based or waterproof stain or paint.

Fire doors

Fire doors are often constructed with layers of fire resistant material that can become damaged if exposed to floodwater. Fire doors that have been immersed in floodwater should always be replaced, together with the door frame seals.


Timber windows

Timber window frames may distort following exposure to floodwater and may need to be re-fitted. As with timber doors such distortion can be minimised by ensuring the timber is effectively sealed using either an oil-based or waterproof stain. Double-glazed aluminium and PVCu windows and patio doors

Modern double-glazed windows and patio doors are normally made from hollow extruded aluminium or PVCu sections. These can fill up with floodwater and can be difficult to drain.

For prolonged floods lasting more than a few hours water can penetrate through the edge seal of double glazed units into the cavity between the glazing. In such cases the glazed units will require replacement.


Solid timber staircases will generally only be slightly affected by flooding. However care should be taken to ensure that the staircase has not become unstable and any loose treads should be replaced.

In some modern housing staircases are formed from MDF and these may need to be re-fitted or replaced following exposure to floodwater. During refurbishment works and for new properties, the installation of solid timber staircases should be considered below the expected flood level.

Skirting boards

Skirting boards are normally formed from softwood timber or MDF. Solid timber skirting boards should not be affected by floodwater provided that all faces, including the back face and underside, are sealed with an oil-based paint or stain before fixing to the wall.

Timber skirting boards will however normally need to be temporarily removed to allow the walls to dry out. Non-treated timber boards will distort following flooding and will usually need to be replaced. Skirting boards formed from MDF are likely to require replacement if immersed in floodwater.

Floor coverings

For properties at risk of flooding fitted carpets should be avoided where possible. Carpets will normally need to be replaced following immersion in floodwater although specialist cleaning services are available.

Consider using loose fitting rugs that can be moved and stored upstairs before flooding occurs. For kitchens and bathrooms, the type of floor covering will depend on the floor type. For solid concrete floors, stone, concrete or ceramic floor tiles should not normally be affected by flooding. However care should be taken to ensure that the tile resin or grout is sufficiently water resistant.

For suspended concrete and timber floors, permanent floor tiles are not usually suitable as they would need to be removed following flooding to gain access to the sub-floor void. Less expensive synthetic floor covers should be considered such as lino etc although these floor coverings will normally need to be replaced following immersion in floodwater. Wooden laminate flooring is also likely to be damaged by floodwater and will normally require replacement.

Wall coverings

Redecoration of the internal walls will almost always be required following flooding. Wallpapers will normally peel away from the wall or become badly stained following immersion in floodwater and will need to be replaced. Hence for properties at risk of flooding it is recommended that papered finishes should be avoided.

Painted walls will normally also need to be redecorated following flooding but will be less expensive to refurbish than papered walls.

The use of low permeability lime-based paints rather than oil based paints or emulsion can allow walls to dry out more quickly following flooding. Ceramic tiling may also be used as discussed above under solid masonry walls but specialist advice should be obtained to ensure that such low permeability finishes do not lead to problems of damp in the walls.

Electricity supply

Before floodwaters arrive the electricity supply should be turned off at the consumer unit (fuse box). Following internal property flooding a qualified electrician should be called to check the buildings electrical system (and any appliances that have been immersed) before it is re-connected. Location of meter and consumer unit (fuse box) For many older properties both the electricity meter and consumer unit (fuse box) are located at low level inside the house, often in the cupboard under the stairs.

If there is sufficient space raising the meter and consumer unit (fuse box) to a higher level above the expected flood level could be considered subject to approval by the local electricity supply company.

For modern houses the electricity meter is often located outside the property in a box mounted on the external wall. For new properties within flood risk areas locating the meter box and internal consumer unit above the expected maximum flood level should be considered.


Modern electrical wiring is not normally affected by immersion in floodwater. However for long duration floods lasting several hours, water may penetrate the insulation and in such cases the wiring may need replacement.

During refurbishment works, or when building new properties, moving the ground floor ring main cables to first floor level could be considered with drop down cables to ground floor sockets. It is also advisable to use plastic cable conduits rather than to plaster cables directly into the wall in order to reduce the cost of future rewiring should this be required. However care must be taken to ensure that cable conduits are installed to avoid low points that could collect water in the event of flooding.

Socket height

For properties at risk of flooding raising the sockets to an appropriate height above flood levels could be considered. Although this could lead to savings in flood repair costs it should be noted that the appliance cables would become more visible inside the room.

Gas supply

In the event of flooding the gas supply should be turned off at the meter. Following immersion by floodwater it is essential for safety reasons that the operation of the gas system and individual gas appliances are inspected by a registered engineer before the system is re-used.

During a flood, water and silt may find its way into gas systems affecting their safe operation. The appliances may light and appear to be working normally but the flue or ventilation systems may have been adversely affected by the floodwater or partially filled with water.

Gas meters may be affected by floodwater and it is worth considering raising meters above the expected flood levels during refurbishment works or for new build properties. Provision should be made for purging gas supply pipes through the installation of appropriate drain points.

Central heating systems


Gas and oil fired boilers and associated pumps and controls should preferably be installed above the maximum expected flood level. Fittings that have been immersed by floodwater should be inspected by a qualified engineer before re-use and may need to be replaced.


Hessian or fibre insulation to pipes below suspended floors can reduce in thickness following soaking leading to a reduction in thermal effectiveness. Pipe insulation below the expected flood level should preferably be replaced with closed cell insulation. When installing new heating systems, pipework routes should be easily accessible to allow pipes to be maintained and washed down following flooding.

Central heating pipework and radiators are unlikely to suffer damage from flooding, unless corrosion occurs as a result of several weeks of contact with water containing salts. This can happen if, for example, pipes are inaccessible within concrete floors.

Storage Heaters

Electrical storage heaters will normally require replacement following immersion in floodwater.

Non-return valves

Flooding can create blockages in drains and sewers which can lead to the backflow of sewage into properties through low level drain gullies, toilets, and washing machine outlets. Backflow through drains is particularly likely where floodwater is prevented from entering the property using temporary barriers and where the external flooding depth outside is above the internal level of the drain entry points.

Such flooding can be effectively controlled by installing non-return valves, often called anti-flooding devices, within the private sewer of a property upstream of the public sewerage system. These devices are typically between 0.5 to 1 metre in length and are installed in-line within an inspection chamber to allow access for maintenance. They are designed for installation within gravity sewers or drains and normally use flap gates to prevent backflow.

It must be remembered that once an anti-flooding device has closed to prevent backflow, it effectively disconnects the property from the public sewerage system.

Appliances within the property, such as toilets, and washing machines cannot be used until the flooding has subsided and the device re-opens, otherwise the property may be flooded by its own discharges. It is important to remember that anti-flooding devices require careful installation and must be regularly maintained. Maintenance depends on the type of device fitted, but is normally required every six months.

 Private sewage systems

Flooding of a private sewage system can create a back up of sewage into the property and lack of sanitation. Damaged septic tanks, cesspools, or small package sewage systems should be inspected by a qualified engineer before the drainage system is re-used.

Water supply

Water meters and pipework will not normally be affected by flooding but some pipework insulation materials may become damaged. This should be replaced with flood resistant closed cell material below the expected flooding level.

Telephone and cable services

Following flooding some problems have been found with communications wiring due to the collection of floodwater within cable conduits. Suppliers of the relevant services should be consulted on suitable installation methods in areas liable to flooding.

Where possible incoming telephone lines, and internal control boxes, should be raised above the expected flood levels.

Where can floodwater enter my property?

Floodwater can find its way into properties through a variety of routes including:

  • Ingress around closed doorways.
  • Ingress through airbricks and up through the ground floor.
  • Backflow through overloaded sewers discharging inside the property through ground floor toilets and sinks.
  •  Seepage through the external walls.
  • Seepage through the ground and up through the ground floor.
  • Ingress around cable services through external walls.

To reduce the likelihood of floodwater entering the property it is important to identify where the potential points of entry are located. The flooding routes will depend on the type of construction, the underlying ground conditions and the expected flood depth.

For example airbricks below ground floor level are normally only found in properties with suspended floors to allow the void under the floor to be ventilated. Other wall vents above the ground floor are likely to be present if gas appliances are in the room.

If sinks and downstairs toilets are located below the expected flood depth it is possible that floodwater will flow back through the sewers and into the property, unless non-return valves are installed.

Seepage through the external walls will depend upon the flooding duration and the type of construction and condition of the wall. Water will tend to find its way through weak points within the wall such as cracks and voids in the mortar jointing, brickwork or rendering.

For semidetached and terraced houses it is important to remember that floodwater may also seep through adjoining party walls with neighbouring properties, above or below floor level. Depending on the permeability of the ground under the property, floodwater may seep through the ground and up through gaps or cracks in floors or other weak points.

The actual rate of seepage is difficult to estimate, as this will depend on the type of ground, the duration of the flooding and the construction of the property.

If your property has flooded in the past then you may already know where the entry points are located but it is recommended that these are reconsidered carefully before any measures to reduce the impact of flooding are taken.

What measures could be taken to improve the flood resistance of the property?

There are many measures that can be taken to reduce the impact of flooding on the home or business property. These measures generally fall within two main categories, those that are taken to keep water out of the building, often called dryproofing measures, and those to improve the ability of the property to withstand the effects of flooding once water has entered the building, known as wetproofing.

Dry-proofing measures include moveable flood protection barriers for doorways, low level windows and other openings that can be installed before the arrival of floodwaters, and the installation of nonreturn valves on sewers to prevent backflow.

Wet-proofing measures include the use of flood resistant building materials within walls and floors and in other parts of the structure, and the raising of electrical wiring above flood levels. The measure or combination of measures chosen will be dependent on the depth and frequency of flooding. The following sections provide more detail on the measures available for both keeping the water out of the property (dry-proofing) and improving the flood resistance of the building elements (wet-proofing). It is recommended that all the dry-proofing and wet-proofing systems mentioned in this guide are considered so that the most appropriate measures for your particular circumstances can be identified.

Cost implications of flood protection measures

The cost of improving the flood protection of a property will vary depending on many factors including the flood depth, property size, construction type, internal finishes, and the location and type of fittings and services. It is therefore difficult to provide any meaningful cost figures within this guidance for comparative purposes.

When considering flood resistance measures the costs of installation needs to be weighed against the potential cost savings that such measures could provide in terms of reduced repair costs, lower temporary accommodation charges, and reduced business losses following future flood events.

In the majority of cases taking steps to improve the flood resistance of properties at risk of flooding will provide long term financial cost savings in addition to reduced levels of stress experienced when flooding occurs.

Phew! That was a long article wasn’t it?!

Surveyor, G A Bell.

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